A confocal optical apparatus, which is an optical apparatus using a confocal optical system, is used as a measuring apparatus for measuring a three-dimensional shape of a surface of a measurement object, or is used as a microscope for measuring the surface of a measurement object. When the confocal optical apparatus is used for these purposes, the confocal optical apparatus needs to be focused at each point on the surface of the measurement object. For this reason, in many cases, the confocal optical apparatus is provided with a focus position changing apparatus for changing the focus position of the objective lens.
Conventionally, as a focus position changing apparatus of this type, a focus position changing apparatus is disclosed in JP-A 9-126739.
The focus position changing apparatus disclosed in JP-A 9-126739 includes a plurality of parallel plate-shaped transparent bodies which are provided in a confocal optical system and different from each other in one of the thickness and the refractive index, and also includes a rotating body on which the plurality of transparent bodies are arranged. The focus position changing apparatus is configured to be able to change the focus position of the objective lens in the optical axis direction (height direction of the measurement object) according to the thickness or the refractive index of the transparent bodies. The transparent bodies are successively made to cross the optical axis in association with the rotation of the rotating body. For this reason, with the focus position changing apparatus using the rotating body of this type, the focus position of the objective lens can be changed precisely and at very high speed as compared with the case where the focus position of the objective lens is fixed and the mounting base of the measurement object is moved.
However, when the focus position changing apparatus using the rotating body is used, the period in which the rotating body itself crosses the optical axis occurs. When the rotating body itself crosses the optical axis, a light beam is reflected by the rotating body. The light beam reflected by the rotating body is originally unnecessary for the confocal imaging system using the light beams reflected by the measurement object, and hence it is desirable to reduce the light beam reflected by the rotating body.
On the other hand, it is preferred that the surface of the rotating body, on which surface the transparent bodies are installed, is formed to have high flatness by polishing or the like of the surface in order to prevent a deviation of the focus position of the objective lens due to an inclination of the transparent body. However, when the flatness of the surface of the rotating body is high, the reflectance of the surface becomes very high. The high reflectance causes increase the amount of light beams which are reflected by the rotating body and inputted into the confocal imaging system.